The present invention relates to a candoluminescent material, which is in particular usable for gas lamp mantles, as well as to its production process.
Since the time of Auer von Welsbach, gas lamp mantles have been made from a combustible fine cloth impregnated with a mineral which forms, after the initial combustion in the gas flame, a netting in the solid state which, in the flame, gives an intense candoluminescence phenomenon. The solid netting rapidly forms as from the initial ignition and acquires its difinitive shape and a solid texture resisting mechanical and thermal shocks. To obtain this resistance, it is preferable for the solid netting to have a fluorine-type cubic crystalline structure. Moreover, for the candoluminescent material to have a high level of brightness in the visible range, it must only have limited emission in the infrared, which makes it possible to reach a high temperature in the substantially colourless flame of the gas.
At present the mantles are constituted by a cloth net impregnated with a mixture of salts, forming after the initial combustion a fine and divided cloth, of thorium oxide containing a little cerium oxide or other oxides, e.g. an oxide containing 99.2 molar % of ThO.sub.2 and 0.8 molar % of CeO.sub.2.
However, the use and production of mantles based on thorium oxide have a certain number of disadvantages. Thus, thorium is a naturally radioactive, alpha-emitting element having a half-life of 1.4.10.sup.10 years and by decay gives various radioactive isotopes, which are alpha, beta or gamma emitters with a short life, including a radioactive gas, thoron 220, which leads to lead 208. Thus, one ton of natural thorium represents about one curie of .sup.232 Th and one curie of .sup.228 Th. However, the thorium quanities used in the production of mantles is by no means negligible in view of the fact that, according to World statistics, the production of candoluminescent mantles is roughly 300 million yearly. Thus, at a rate of 0.3 g of thorium per mantle, the thorium quantity involved is 100 tons yearly, which leads to a dissemination of thorium.
Thus, when the mantles are broken, the impalpable thorium oxide powder is disseminated or scattered in the environment. Moreover, it is usually necessary to maintain a stock of of several dozen kilograms of thorium, which produces radioactive thoron, so that certain precautions should be taken in connection with the handling, storage and transportation of these mantles. Finally, in view of the fact that thorium can be used as a nuclear fuel in fast neutron reactors it would be preferable to reserve it for this use and use other materials for producing gas lamp mantles.
With this aim, various products have been proposed, such as alumina based materials doped with other oxides, but the results obtained with them have not been satisfactory.